By Warren Upham
Secretary of the Minnesota Historical Society, Formerly Assistant
on the Geological Surveys of New Hampshire, Minnesota,
the United States, and Canada.
The Red River of the North, so named to distinguish it from the Red river of Louisiana, flows through an exceedingly flat plain, which descends imperceptibly northward, as also from each side to its central line. Along the axial depression the river has cut a channel twenty to sixty feet deep. It is bordered by only few and narrow areas of bottomland, instead of which its banks usually rise steeply on one side, and by moderate slopes on the other, to the broad valley plain which thence reaches nearly level ten to twenty-five miles from the river. Its tributaries cross the plain in similar channels, which, as also the Red river, have occasional gullies connected with them, dry through most of the year, varying from a few hundred feet to a mile or more in length. Between the drainage lines, areas often five to fifteen miles wide remain unmarked by. any water courses. The highest portions of these tracts are commonly from two to five feet above the lowest.
This vast plain, twenty-five to fifty miles wide and 300 miles long, lying half in Minnesota and half in North Dakota, thence continuing into Manitoba and so stretching from Lake Traverse and Breckenridge north to Lake Winnipeg, is the widely famed Red River valley. The material of the lower part of the valley plain, shown in the banks of the Red river and reaching usually five to fifteen miles from it, is fine clayey silt, horizontally stratified; but at its south end, in Traverse county and the south half of Wilkin county, Minnesota, through the adjoining part of Richland county, North Dakota, and upon large areas of each side of this plain, it is mainly unstratified boulder clay, which differs from the rolling or undulating till of the adjoining region only in having its surface nearly flat. Both these formations are almost impervious to water, which, therefore, in the rainy season fills their shallow depressions, but none of these are so deep as to form permanent lakes. Even sloughs which continue marshy through the summer are infrequent, but where they do occur, as on some of the streams tributary to the Red river, they cover large areas, sometimes several miles in extent.
In crossing this almost perfectly level valley on clear days, the higher land at its sides, and the groves along its rivers, are first seen in the distance as if their upper edges were raised a little above the horizon, with a very narrow strip of sky below. The first appearance of the tree tops thus somewhat resembles that of dense flocks of birds flying very low several miles away. By rising a few feet, as from the ground to a wagon, or by nearer approach, the outlines become clearly defined as a grove, with a mere line of sky beneath it.
Besides this mirage, the traveler is also reminded, in the same manner as at sea, that the earth is round. The surface of the plain is seen only for a distance of three or four miles; houses and grain stacks have their tops visible first, after which, in approaching, they gradually come into full view; and the highlands, ten or fifteen miles away, forming the side of the valley, apparently lie beyond a wide depression, like a distant high coast.
On nearly all the area drained by the Red river the glacial drift is so thick that no exposures of the underlying rocks have been found. Along the flat valley plain, the average depth of the drift is from about 100 to 200 feet. The prominent topographic features of all this region are doubtless due to the form of the underlying rock surface, upon which the drift is spread in a sheet of somewhat uniform thickness. Subaerial denudation and stream erosion, during the Tertiary era and the early part of Quaternary time, preceding the Ice Age, had sculptured this broad and flat valley trough and the inclosing uplands which on each side gradually rise 200 to 500 feet above the valley.
Lakes in northern and central Becker county, Minnesota, forming the sources of Otter Tail river, the head stream of the Red river, are 1,400 to 1,500 feet above the sea; Otter Tail lake, 1,315 feet; Lake Clitherall, 1,334; and the East and West Battle lakes, 1,328. The Red river at Fergus Falls descends about eighty feet in three miles, from 1,210 to 1,130 feet; at Breckenridge and Wahpeton its height at the stage of low water is 943 feet; at Moorhead and Fargo, 866 feet; at Grand Forks, 784; at St. Vincent and Pembina, 748; and at the city of Winnipeg, 724 feet above the sea.
The range between the lowest and highest stages of the Red river much surpasses that of any other river in Minnesota or North Dakota. At Breckenridge and Wahpeton the range is about fifteen feet, but it increases rapidly northward, becoming thirty-two feet at Moorhead and Fargo, attaining its maximum of fifty feet near the mouth of the Sand Hill river in the south part of Polk county, Minnesota, and continuing nearly at forty feet from Grand Forks to the international boundary and Winnipeg. Floods rising nearly or quite to the highwater line thus noted have been rare, occurring in 1826, 1852, 1860, 1861, and 1882. They are caused in the spring by the melting of unusual supplies of snow and by heavy rains, and often are increased by gorges of ice, which is usually broken up along the southern upper portion of the river earlier than along its lower course. These floods attain a height only a few feet below the level of the adjoining prairie where that is highest, and along the greater part of the distance between Fargo and Winnipeg the banks are overflowed and the flat land on each side of the river to a distance of two to four miles from it is covered with water one to five feet or more in depth.
The Archean Era
Granite, syenite, greenstone, gneiss, and schists, belonging to the Archean or Beginning era, reach on the northern boundary of Minnesota from Gunflint and Saganaga lakes west to the Lake of the Woods. They thence extend south upon a large part of St. Louis and Itasca counties to the Vermilion and Mesabi ranges, famed for their immense deposits of iron ore.
A narrow Archean belt continues from this great area southward, mostly covered by the glacial drift, and expands into a second large area of these rocks in central Minnesota, reaching from Todd, Morrison and Stearns counties northeast to Carlton county and south to New Ulm. The extensive granite quarries near St. Cloud and Sauk Eapids are in this area.
The same rocks also underlie a large district west of New Ulm, extending to the western boundary of Minnesota, mainly covered by Cretaceous beds and glacial drift. In that part of the Minnesota River valley, channeled about 150 feet below the general level of the country, the Archean granites and gneisses are seen in many and extensive outcrops, and have been much quarried at Ortonville, near the mouth of Big Stone lake.
Archean time, during which these oldest rocks were formed, was exceedingly long, perhaps equaling all the later eras. Its early part may be termed azoic, from the absence of any evidences that the earth or the sea then had either plant or animal life.
Next after the Archean was a very long era characterized by ancient types of life, as its name Paleozoic signifies. The chief divisions of this era have been named by geologists the Cambrian, Silurian, Devonian, Carboniferous, and Permian periods, succeeding each other in this order.
In journeying from south to north along the Red River valley, the first rock exposures found are Lower Silurian strata, chiefly magnesian limestones, which outcrop in Manitoba at numerous localities twelve to twenty miles north-northeast of Winnipeg, and similar outcrops, probably in part of Upper Silurian age, which rise above the general surface of drift five to twenty miles northwesterly from Winnipeg and at about the same distance west of the river. Farther north, Lower Silurian rocks are exposed on many of the islands of Lake Winnipeg and along its western shore, but no exposures of the underlying Cambrian beds, which are penetrated by the artesian well at Grafton, North Dakota, have been found in this region. Against the western border of the folded and eroded Archean rocks the Lower Silurian formations repose with nearly horizontal stratification. Their general dip, varying from a few feet to ten feet or more per mile, is westward, at right angles with the axis of Lake Winnipeg and the line of junction of the Archean and Paleozoic rocks.
West of these Lower Silurian strata, rocks of Devonian age, mostly pale-gray or bluff magnesian limestones, occur on Lakes Manitoba and Winnipegosis, as reported in 1884 by Dr. George M. Dawson; ”and it is probable,” he wrote, “that the intervening formations will be found to be extensively developed in the Lake Winnipeg region as it is more fully examined.”
Subsequent exploration of this region by Mr. J. B. Tyrrell resulted in the discovery of Upper Silurian strata, containing fossils characteristic of the Niagara formation, on the lower part of the Saskatchewan river and on the east side of Lakes Manitoba and Winnipegosis. All the Paleozoic formations in the lake region of Manitoba, from the St. Peter sandstone to the highest Devonian beds exposed, are stated by Mr. Tyrrell to be “practically conformable and almost undisturbed throughout.”
This region has no Carboniferous nor Permian strata, belonging to the closing periods of Paleozoic time. If any sediments were then laid down here, they have since been eroded and removed during long ensuing ages, when the basin of the Red river was a land surface. Probably it stood above the sea, receiving no marine nor estuarine deposits, but undergoing slow erosion by rains, rills, and rivers, bearing sediments away, during the Carboniferous period and onward until the Cretaceous period.
Through the early and greater part of the Mesozoic era, so named from its intermediate types of plants and animals, this river basin appears to have been a land area, receiving therefore no additions to its rock formations. The floras and faunas of this time were gradually changed from their primitive and ancient characters, called Paleozoic, but had not yet attained to the relatively modern or new forms which give the name Cenozoic to the next era.
Toward the end of the Cretaceous period, in late Mesozoic time, this area was again mostly depressed beneath the sea. Frequent outcrops of Cretaceous shales and sandstone, continuous from their great expanse on the western plains, occur in some parts of central and southern Minnesota; and in numerous other places, deep wells, after passing through the thick covering of glacial drift, encounter these Cretaceous strata, which sometimes are found to reach to a thickness of several hundred feet. Further evidence of the eastward extension of the Cretaceous sea upon this state is afforded in its northern part by Horace V. Winchell’s discoveries of Cretaceous shales in place on the Little Fork of the Rainy river and on the high Mesabi iron range.
During the following Cenozoic era, when this was a land region subjected to erosion, its Cretaceous deposits were largely carried away; but a remaining portion, in some tracts having considerable depth, probably still lies beneath the drift on the greater part of the western four-fifths of Minnesota. Concerning its eastern limit, Professor N. H. Winchell writes: “A line drawn from the west end of Hunter’s Island, on the Canadian boundary line, southward to Minneapolis, and thence southeastwardly through Rochester to the Iowa state line, would, in general, separate that part of the state in which the Cretaceous is not known to exist from that in which it does. It is not here intended to convey the idea that the whole state west of this line is spread over with the Cretaceous, because there are many places where the drift lies directly on the Silurian or earlier rocks; but throughout this part of the state the Cretaceous exists at least in patches, and perhaps once existed continuously.”
Farther north, along the west line of the lower part of the Red River valley and of Lakes Manitoba and Winnipegosis, Cretaceous beds rest upon the Lower and Upper Silurian and Devonian strata that form the floor of this broad, flat valley, beneath its glacial, lacustrine, and fluvial deposits. Thence northwestward to the Mackenzie river and the Arctic ocean, Cretaceous formations border and overlie the west part of the Silurian and Devonian belt. West from the Red river, the Cretaceous area in North Dakota and Montana, and in the Dominion of Canada, has a width of 600 to 700 miles, including the entire region of the elevated plains, and terminating at the east base of the Rocky mountains.
Ever since the uplift of the Red River basin from the Cretaceous sea, it has stood above the sea level and has received no marine sediments. It was instead being slowly sculptured by rains and streams through the long periods of the Tertiary era; and during a part of the relatively short Quaternary era it was deeply covered by snow and ice similar to the ice-sheets that now envelop the interior of Greenland and the Antarctic continent.
These two eras, or principal divisions of geologic history, may be here classed together as a single Cenozoic era, distinguished by the evolutionary creation of new and present types of life. Nearly all the plants and animals of the preceding eras have disappeared, as also many that lived in the early Cenozoic periods, while new species succeeding them make up the present floras and faunas.
The creation of man, his dispersion over the earth, and his development in the white, black, yellow, and red races, took place during the later part of Cenozoic time, which is often called the Pleistocene (meaning the newest) period or the Quaternary era. Finally the dominance of mankind in the history of the earth, with utilization of its vast resources, forms another grand time division which has been called the Psychozoic era, distinguished by the higher life and dominion of the mind or soul. Thus the Tertiary, Quaternary, and Psychozoic divisions of time are successive parts of the Cenozoic era, continuing to the present day.
Kains, rills and rivulets, creeks and rivers, have been slowly but constantly wearing away the Cretaceous formations of the Northwest since their elevation above the sea and the drainage of the immense Laramie lake, which for a long period covered much of their area. When these marine and lacustrine deposits were first raised to be dry land, they had a monotonously flat surface; and they probably extended east, as we have seen, over the entire basin of the Bed River of the North and of the great lakes of Manitoba, from which they now reach to the Rocky mountains. The greater part of the present Cretaceous area, though eroded far below its original surface, is flat, undulating, or only moderately rolling, and constitutes a broad expanse of plains with very slow ascent westward. But here and there isolated areas of much higher hilly land, as the Turtle mountain, consist of remnants of horizontal Cretaceous strata which elsewhere have suffered denudation over all the surrounding country. The plains have been formed by the erosion of this vast area to a uniform base-level, excepting only the isolated hilly tracts of comparatively small extent, which serve to show that on the eastern part of the plains, in North Dakota and southwestern Manitoba, a thickness of not less than 500 to 1,000 feet of the Laramie, Fox Hills, and Fort Pierre formations has been carried away.
When the depth and great extent of this denudation are compared with those of the subsequent erosion which formed the Red River valley and the lowland adjoining the Manitoba lakes by the removal of the former eastern part of the Cretaceous plains to the limit of the great escarpment west of this valley, the early base-leveling seems probably to have occupied the Eocene and Miocene periods, with nearly all of the Pliocene, comprising nine-tenths or a longer portion of the whole Tertiary era.
At the time of the later uplifting of the plains near the end of the Pliocene period, this great base-leveled region appears to have stretched from the Rocky mountains to the Archean hills of northern Minnesota, and to have included also the expanse of flat or only moderately undulating country which slowly falls from Lake Winnipeg and the upper part of the Nelson river toward Hudson bay.
The eastern margin of these plains was then subjected to renewed erosion, removing the mostly soft Cretaceous strata upon a width of a hundred miles or more and to a depth westward of several hundred feet. Previous to this new cycle of active work by the streams, Riding and Duck mountains in Manitoba stood above the general level, like Turtle mountain and other isolated high areas farther west; and the maximum depth of the late stream-cutting by which the trough of the Red River valley was formed is approximately measured by the height of the Pembina Mountain escarpment, which rises 300 to 400 feet from its base to its crest along its extent of about 80 miles. The greater part of this erosion we must attribute to the probably long time of elevation preceding, and finally at its climax producing, the ice-sheet of the Glacial period. So far as can be discerned, the entire hydrographic basin of the Red river may have continued, through all these vicissitudes of changes of level, excepting when it was wholly or partially ice-covered, to be drained in the same north and northeast direction as during the Tertiary era and at the present day.
Tertiary and early Quaternary erosion had sculptured the grand features of this river basin, and its whole extent probably had approximately the same contour immediately before the accumulation of the ice-sheet as at the present time. The surface of the feldspathic Archean rocks was doubtless in many places decomposed and kaolinized as it is now seen where they are uncovered in the Minnesota River valley, and as such rocks are frequently changed to a considerable depth in regions that have not been glaciated. On these and all the other rock formations the ordinary disintegrating and eroding agencies of rain and frost had been acting through long ages. Much of the loose material thus supplied had been carried by streams to the sea, but certainly much remained and was spread in general with considerable evenness over the surface, collecting to the greatest depth in valleys, while on ridges or hilltops it would be thin or entirely washed away. Except where it had been transported by streams and consequently formed stratified deposits, the only fragments of rock held in this mass would be from underlying or adjoining rocks. The surface then probably had more small inequalities than now, due to the irregular action of the processes of weathering and denudation, which are apt to spare here and there isolated cliffs, ridges, and hillocks; but most of these minor features of the topography have been obliterated by glacial erosion or buried under the thick mantle of the drift.
The Ice Age
The last among the completed periods of geology was the Ice age, most marvelous in its strange contrast with the present time, and also unlike any other period during the almost inconceivably long, uniformly warm or temperate eras which had preceded. The northern half of North America and northern Europe then became enveloped with thick sheets of snow and ice, probably caused chiefly by uplifts of the land as extensive high plateaus, receiving snowfall throughout the year. But in other parts of the world, and especially in its lower temperate and tropical regions, all the climatic conditions were doubtless then nearly as now, permitting plants and animals to survive and nourish until the departure of the ice-sheets gave them again opportunity to spread over the northern lands.
High pre-glacial elevation of the drift-bearing regions is known by the depths of fjords and submerged continuations of river valleys, which on the Atlantic, Arctic, and Pacific coasts of the north part of North America show the land to have been elevated at least 2,000 to 3,000 feet higher than now. In Norway the bottom of the Sogne fjord, the longest and deepest of the many fjords of that coast, is 4,000 feet below the sea level. Previous to the Glacial period or Ice age, and doubtless causing its abundant snowfall, so high uplift of these countries had taken place that streams flowed along the bottoms of the fjords, channeling them as very deep gorges on the borders of the land areas.
Under the vast weight of the ice-sheets, however, the lands sank to their present level, or mostly somewhat lower, whereby the temperate climate, with hot summers, properly belonging to the southern portions of the ice-clad regions, was restored. The ice-sheets were then rapidly melted away, though with numerous pauses or sometimes slight re-advances of the mainly receding glacial boundary.
On certain belts the drift was left in hills and ridges, accumulated during this closing stage of the Glacial period along the margin of the ice wherever it halted in its general retreat or temporarily re-advanced. Upon the greater part of Minnesota and North Dakota the only hills are formed of this morainic drift, ranging in height commonly from 25 to 75 to 100 feet, but occasionally attaining much greater altitude, as in the Leaf hills of Otter Tail county, Minnesota, which rise from 100 to 350 feet above the moderately undulating country on each side.
Unstratified glacial drift, called till or boulder clay, which was laid down by the ice-sheet without modification by water transportation, assorting, and deposition in beds, forms the surface of probably two-thirds, or a larger part, of these states and of Manitoba. It consists of boulders, gravel, sand, and clay, mingled indiscriminately together in a very hard and compact formation, which therefore is frequently called “hardpan.” The boulders of the till are usually so plentiful that they are sprinkled somewhat numerously on its surface; yet there are seldom more, on the large portions of the country which are adapted for agriculture, than the farmer needs to use, after clearing them from his fields, for the foundations of buildings and for walling up his cellar and well. They are rarely abundant enough to make walls for the enclosure of fields, as in New England.
The moraine belts of knolly and hilly till have far more abundant boulders than are found on its more extensive comparatively smooth tracts. Wherever the vicissitudes of the wavering climate caused the chiefly waning border of the ice-sheet to remain nearly stationary during several years, the outflow toward the melting steep frontal slope brought much drift which had been contained in the lower part of the ice, heaping it finally in hills and ridges along the ice margin. Twelve of these marginal belts of drift knolls and hills have been traced in irregularly looped courses across Minnesota, as described and mapped in the reports of that state; and west of the Red River valley these knolly drift belts continue through the northeastern half of North Dakota, and onward across the international boundary.
About a third part of the entire mantle of drift consists of the deposits called modified drift, being waterworn and stratified gravel, sand, and clay or silt, which were washed away from the drift upon and beneath the retreating ice-sheet by the streams due to its melting and to accompanying rains. Hillocks and ridges of gravel and sand (called kames and eskers), sand plateaus and plains, and the valley drift (varying from very coarse gravel to very fine clay, often eroded so that its remnants form terraces), are the principal phases of the modified drift. In being derived directly from the ice-sheet, these deposits had the same origin as the glacial drift forming the common till and the greater part of the marginal moraines; but they were modified, large boulders being not included, while the gravel and finer portions were brought, further pulverized or rounded, and assorted in layers, by water.
Glacial Lake Agassiz
When the departing ice-sheet, in its melting off the land from south to north, receded beyond the watershed dividing the basin of the Minnesota river from that of the Red river, a lake, fed by the glacial melting, stood at the foot of the ice fields, and extended northward as they withdrew along the valley of the Red river to Lake Winnipeg, filling this broad valley to the height of the lowest point over which an outlet could be found. Until the ice barrier was melted on the area now crossed by the Nelson river, thereby draining this glacial lake, its outlet was along the present course of the Minnesota river. At first its overflow was on the nearly level undulating surface of the drift, 1,100 to 1,125 feet above the sea, at the west side of Traverse and Big Stone counties; but in the process of time this cut a channel there, called Brown’s Valley, 100 to 150 feet deep and about a mile wide, the highest point of which, on the present water divide between the Mississippi and Nelson basins, is 975 feet above the sea level. From this outlet the valley plain of the Red river extends 315 miles north to Lake Winnipeg, which is 710 feet above the sea. Along this entire distance there is a very uniform continuous descent of a little less than one foot per mile.
The farmers and other residents of this fertile plain are well aware that they live on the area once occupied by a great lake, for its beaches, having the form of smoothly rounded ridges of gravel and sand, a few feet high, with a width of several rods, are observable extending horizontally long distances upon each of the slopes which rise east and west of the valley plain. Hundreds of farmers have located their buildings on these beach ridges as the most dry and sightly spots on their land, affording opportunity for perfectly drained cellars even in the most wet spring seasons, and also yielding to wells, dug through this sand and gravel, better water than is usually obtainable in wells on the adjacent clay areas. While each of these farmers — in fact, everyone living in the Red River valley — recognizes that it is an old lake bed, few probably know that it has become for this reason a district of special interest to geologists, who have traced and mapped its upper shore along a distance of about 800 miles. Numerous explorers of this region, from Long and Keating in 1823, to General G. K. Warren in 1868 and Professor N. H. Winchell in 1872, recognized the lacustrine features of this valley; and the last-named geologist first gave what is now generally accepted as the true explanation of the lake’s existence, namely, that it was produced in the closing stage of the Glacial period by the dam of the continental ice-sheet at the time of its final melting away. As the border of the ice-sheet retreated northward along the Red River valley, drainage from that area could not flow, as now, freely to the north through Lake Winnipeg and into the ocean at Hudson bay, but was turned by the ice barrier to the south across the lowest place on the watershed, which was found, as before noted, at Brown’s Valley, on the west boundary of Minnesota.
Detailed exploration of the shore lines and area of this lake was begun by the present writer for the Minnesota Geological Survey in the years 1879 to 1881. In subsequent years I was employed also in tracing the lake shores through North Dakota for the United States Geological Survey, and through southern Manitoba, to the distance of 100 miles north from the international boundary, for the Geological Survey of Canada. For the last-named survey, also, Mr. J. B. Tyrrell extended the exploration of the shore lines, more or less completely, about 200 miles farther north, along the Riding and Duck mountains and the Porcupine and Pasquia, hills, west of Lakes Manitoba and Winnipegosis, to the Saskatchewan river.
This glacial lake was named by the present writer in the eighth annual report of the Minnesota Geological Survey, for the year 1879, in honor of Louis Agassiz, the first prominent advocate of the theory of the formation of the drift by land ice. Its overflowing river, whose channel is now occupied by Lakes Traverse and Big Stone and Brown’s Valley, was also named by me, in a paper read before the American Association for the Advancement of Science, at its Minneapolis meeting in 1883, as the River Warren, in commemoration of General Warren’s admirable work in the United States Engineering Corps, in publishing maps and reports of the Minnesota and Mississippi river surveys. Descriptions of Lake Agassiz and the River Warren were somewhat fully given in the eighth and eleventh annual reports of the Minnesota Geological Survey, and in the first, second and fourth volumes of its final report; and more complete descriptions and maps of the whole lake, in Minnesota, North Dakota and Manitoba, were published in 1895 as Monograph XXV of the United States Geological Survey.
Several successive levels of Lake Agassiz are recorded by distinct and approximately parallel beaches of gravel and sand, due to the gradual lowering of the outlet by the erosion of the channel at Brown’s Valley, and these are named principally from stations on the Breckenridge and Wahpeton line of the Great Northern railway, in their descending order, the Herman, Norcross, Tintah, Campbell, and McCauleyville beaches, because they pass through or near these stations and towns. The highest or Herman beach is traced in Minnesota from the northern end of Lake Traverse eastward to Herman, and thence northward, passing a few miles east of Barnesville, through Muskoda, on the Northern Pacific railway, and around the west and north sides of Maple lake, which lies about twenty miles east-southeast of Crookston, beyond which it goes eastward to the south side of Red and Rainy lakes. In North Dakota the Herman shore lies about four miles west of Wheatland, on the Northern Pacific railway, and the same distance west of Larimore, on the Pacific line of the Great Northern railway. On the international boundary, in passing from North Dakota into Manitoba, this shore coincides with the escarpment or front of the Pembina Mountain plateau; and beyond passes northwest to Brandon on the Assiniboine, and thence northeast to the Riding mountain.
Leveling along the upper beach shows that Lake Agassiz, in its earliest and highest stage, was nearly 200 feet deep above Moorhead and Fargo; a little more than 300 feet deep above Grand Forks and Crookston; about 450 feet above Pembina, St. Vincent, and Emerson : and about 500 and 600 feet, respectively, above Lakes Manitoba and Winnipeg. The length of Lake Agassiz is estimated to have been nearly 700 miles, and its area not less than 110,000 square miles, exceeding the combined areas of the five great lakes tributary to the St. Lawrence.
After the ice border was so far melted back as to give outlets northeastward lower than the River “Warren, numerous other beaches marking these lower levels of the glacial lake were formed; and finally, by the full departure of the ice, Lake Agassiz was drained away to its present representative, Lake Winnipeg.
While the outflow passed southward, seventeen successive shore lines, marked by distinct beach ridges, were made by the gradually falling northern part of this lake; but all these, when traced southward, are united into the five beaches before noted for the southern part of the lake. During its stages of northeastern outflow, a lower series of fourteen shore lines were made. Thus Lake Agassiz had, in total, thirty-one successive stages of gradual decline in height and decrease in area.
The earliest Herman beach has a northward ascent of about a foot per mile, but the lowest and latest beaches differ only very slightly from perfect horizontality. It is thus known that a moderate uplift of this area, increasing in amount from south to north, was in progress and was nearly or quite completed while the ice-sheet was melting away. Before the Glacial period, all the northern half of our continent had been greatly elevated, producing at last the cold and snowy climate and the thick ice-sheet; in a late part of that period the land was depressed under the weight of the ice, which in consequence melted away; and latest, at the same time with the departure of the ice-sheet, the unburdened land rose a few hundred feet, the uplift having a gradual increase toward the central part of the country formerly ice-covered.
In comparison with the immensely long and ancient geologic periods that had preceded, the final melting of the ice-sheet, the deposition of its marginal moraines and other drift formations, its fringing glacial lakes, and the attendant uplifting of the land, occupied little time and were very recent. The entire duration of Lake Agassiz, estimated from the amount of its wave action in erosion and in the accumulation of beach gravel and sand, appears to have been only about 1,000 years, and the time of its existence is thought to have been somewhere between 6,000 and 10,000 years ago.
Length of Time Since the Ice Age
In various localities we are able to measure the present rate of erosion of gorges below waterfalls, and the length of the post-glacial gorge divided by the rate of recession of the falls gives approximately the time since the end of the Ice Age, and since the geologically brief existence of this great glacial lake. Such measurements of the gorge and Falls of St. Anthony on the Mississippi river at Minneapolis by Professor N. H. Winchell show the length of the Postglacial or Recent period to have been about 8,000 years; and from the surveys of Niagara Falls, Professor G. F. Wright and the present writer believe it to have been 7,000 years, more or less. From the rates of wave-cutting along the sides of Lake Michigan and the consequent accumulation of sand around the south end of the lake, Dr. E. Andrews estimates that the land there became uncovered from its ice-sheet not more than 7,500 years ago. Professor “Wright obtains a similar result from the rate of filling of kettle-holes among gravel knolls and ridges, and likewise from the erosion of valleys by streams tributary to Lake Erie; and Professor B. K. Emerson, from the rate of deposition of modified drift in the Connecticut valley at Northampton, Mass., thinks that the time since the Glacial period cannot exceed 10,000 years. An equally small estimate is also indicated by the studies of Gilbert and Russell for the time since the highest rise of the Quaternary lakes, Bonneville and Lahontan, lying in Utah and Nevada, within the Great Basin of interior drainage, which are believed to have been contemporaneous with the great extension of ice-sheets upon the northern part of our continent.
Professor James Geikie maintains that the use of paleolithic implements in the Stone Age had ceased, and that early man in Europe made neolithic (polished) implements, before the recession of the ice-sheet from Scotland, Denmark, and the Scandinavian peninsula; and Prestwich suggests that the dawn of civilization in Egypt, China, and India may have been coeval with the glaciation of northwestern Europe. In Wales and Yorkshire the amount of denudation of limestone rocks on which boulders lie has been regarded as proof that a period of not more than 6,000 years has elapsed since the boulders were left in their positions. The vertical extent of this denudation, averaging about six inches, is nearly the same with that observed in the southwest part of the province of Quebec by Sir William Logan and Dr. Robert Bell, where veins of quartz worn by glaciation stand out to various heights not exceeding one foot above the weathered surface of the inclosing limestone.
From this wide range of concurrent but independent testimonies, we may accept it as practically demonstrated that the period since the ice-sheets disappeared from North America and Europe, and also since Lake Agassiz existed in the Red River valley, measures some 6,000 to 10,000 years. Within this period are comprised the successive stages of man’s development of the arts, from the time when his best implements were polished stone, through ages of bronze, iron, and finally steel, to the present time, when steel, steam, and electricity bring all nations into close alliance.